(a) Field of the Invention
The present invention relates to a power Off upshift control method for an automatic transmission, and more particularly, to a control method for an automatic transmission capable of reducing shift shock caused by engine rpm reduction and hydraulic response delay.
(b) Description of the Related Art
In an automatic transmission, gear shifting is generally performed in such a way that a transmission control unit TCU controls a plurality of solenoid valves for guiding hydraulic pressure so as to apply and release the pressure to and from specific friction elements, resulting in obtaining a target gear ratio in the transmission.
That is, if a shift lever is manipulated to indicate a target speed range, a manual valve in a hydraulic system responsively converts its ports to a corresponding speed range arrangement such that hydraulic pressure from an oil pump is distributed to expected hydraulic lines. The distributed pressure is selectively supplied to various friction elements via various valves under control of the TCU.
Thus, the shift performance of the automatic transmission is determined in accordance with how timely corresponding friction elements response to the shift lever manipulation.
The different types of shift control include power On/Off upshifting during which the shift takes place in a sequential fashion from 1 to 4 in each state whether an accelerator pedal is depressed or not, power On/Off downshifting during which the shift take place in a sequential fashion from 4xe2x86x921 in each state whether the accelerator pedal is depressed or not, and skip shifting such as 4xe2x86x922 and 3xe2x86x921 shifts.
Among the above shift types, the present invention relates to the power Off upshift control.
If there is a shift lever manipulation for power Off upshift, for example 1xe2x86x922 shift, the shift is conventionally performed in accordance with a predetermined shift pattern stored in a memory of the TCU.
In the conventional power Off upshift control method, since compensation for engine rpm reduction and response time delay of the friction element is not considered, shift shock is experienced during the shift operation.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a power Off upshift control method for an automatic transmission capable of reducing shift shock by compensating for the engine rpm reduction and response time delay of the friction elements using learning control.
To achieve the above object, the power Off upshift control method of the present invention comprises the steps of a) determining if a vehicle is under conditions requiring power Off upshift control; b) starting power Off upshift duty control if the conditions are satisfied; c) calculating average turbine rpm change rate; d) measuring turbine rpm changing period and determining if the turbine rpm changing period is greater than a predetermined value; e) calculating a shift finish point if the turbine rpm changing period is greater than the predetermined value; f) determining if the turbine rpm change rate is less than a value obtained by adding a mapping value to the turbine rpm change rate; g) determining if the turbine rpm value is greater than or equal to the shift finish point, if the turbine rpm change rate is greater than the mapped value in the previous step; h) producing a shift finish point duty rate if the turbine rpm value is greater than or equal to the shift finish point; i) determining if the turbine rpm is less than a target turbine rpm; j) ending duty control if the turbine rpm is less than the target turbine rpm; k) determining if the turbine rpm is less than the a value obtained by subtracting 100 rpm from the target turbine rpm; and l) compensating duty rate and learn the duty rate if the turbine rpm is less than the subtracted value in the previous step.